1. Field of the Invention
The invention generally relates to receivers in spread spectrum communications systems such as WLAN (Wireless Local Area Network) systems, and in particular to the phase error correction of received signals.
2. Description of the Related Art
A Wireless Local Area Network is a flexible data communications system implemented as an extension to or as an alternative for, a wired LAN. Using radio frequency or infrared technology, WLAN devices transmit and receive data over the air, minimizing the need of wired connections. Thus, WLAN systems combine interconnectivity with user mobility.
Most WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in reliable and secure communications systems. The spread spectrum technology is designed to trade-off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
The standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum, is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to the 802.11b standard that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. This extension is backwards compatible as far as it relates to direct sequence spread spectrum technology, but it adopts a new modulation technique called CCK (Complementary Code Keying) which allows the speed increase.
In WLAN systems as well as in other spread spectrum communication systems, the signal on its way from the transmitter to the receiver experiences several distortions. A frequency error may result from a frequency offset of the radio frequency oscillators at the transmitter and the receiver.
Assuming s(t) to be the transmitted signals(t)=A(t)·ejωtwhere ω is the carrier frequency, the received signal can be described asr(t)=B(t)·ej[(ω+ωe)t+φe(t)]where ωe is the oscillator frequency difference between receiver and transmitter, and φe is the difference in oscillator phase between the receiver and the transmitter.
Turning now to FIG. 1, an error correction arrangement is schematically shown consisting of a frequency error correction unit 100 and a phase error correction unit 110. The frequency error correction unit 100 is used to compensate for the frequency difference, and the phase error correction unit 110 will then compensate for the residual phase error. This will now be described in more detail.
Assuming the baseband signal input to the frequency error correction unit 100 be given asB(t)·ej(ωet+φ0)the output signal of the frequency error correction unit 100 will beB(t)·ej({tilde over (ω)}et+φ0)where {tilde over (ω)}e denotes the residual frequency error. This signal can be considered a signal with time dependent phaseφe(t)={tilde over (ω)}et+φ0which will linearly progress in time, as {tilde over (ω)}e and φ0 are constant values.
The phase error correction unit 110 has now the task to remove the remaining phase error such that the received signal is as close as possible to the transmitted signal, to minimize the probability of demodulation errors. An example of how the phase error correction unit 110 may operate is depicted in FIG. 2.
The phase error correction unit shown in FIG. 2 includes an error correction module 200 that performs the following operation:B(t)·ejφe(t)·e−j{tilde over (φ)}e(t)=B(t)·ej[φe(t)−{tilde over (φ)}e(t)]where {tilde over (φ)}e(t) is the current estimate of the phase error. The error correction module 200 is controlled by means of an error signal received from the measurement module 210. The measurement module 210 measures the phase error of the output signal of the correction module 200 and tries to generate the error signal so as to minimize the phase difference φe(t)−{tilde over (φ)}e(t).
Turning now to FIG. 3, the state diagram (or constellation diagram) for a BPSK (Binary Phase Shift Keying) system is shown. The diagram has been rotated for explanatory reasons. In the diagram, the hollow symbols represent the “ideal” signal points whereas the cross mark represents the signal point of the received signal which is phase offset. The current phase difference between the ideal and the received constellation point is given by Δ{tilde over (φ)}e(t). It is the task of the error measurement module 210 to determine this phase difference to generate the error signal as precise as possible.
Let x(k),x(k−1),x(k−2), . . . be the real parts of the received data samples, and y(k),y(k−1),y(k−2), . . . the respective imaginary parts, and let the real and imaginary parts of the ideal constellation point be given by xA and yA, respectively, the phase error can then be calculated according to
                                          Δ            ⁢                                                  ⁢                                                            φ                  ~                                e                            ⁡                              (                t                )                                              =                                    arctan              ⁡                              (                                                      y                    A                                                        x                    A                                                  )                                      -                          arctan              ⁡                              (                                                      y                    ⁡                                          (                      t                      )                                                                            x                    ⁡                                          (                      t                      )                                                                      )                                                    ⁢                                                          t        =                  {                      T            ,                          2              ⁢              T                        ,                          3              ⁢              T                        ,            …                    ⁢                                          }                                                  T          =                                    1              11                        ⁢                                                  ⁢            MHz                          ⁢                                      
However, there is always an additive white Gaussian noise in the received signal so that the measured signal point will deviate from the cross mark shown in FIG. 3 randomly. To illustrate this, there is shown in FIG. 3 a range around the cross mark indicating the region where the measured constellation points will be randomly distributed with a certain probability, due to the additive noise. The region is shown to have a certain radius, and this radius will depend on the current channel conditions in the communication system.
Thus, measuring the phase difference as shown above has the disadvantage that due to the additive noise, there will be a random measurement error. The greater the radius of the noise region, the greater will be the measurement error. It is to be noted that the measurement error may be up to 100% if the distance between the ideal signal point and the received signal point, i.e. the cross mark, in the constellation diagram does not exceed the radius of the noise region.
Evidently, the measurement module 210 cannot accurately generate an error signal if the phase difference cannot be measured precisely. Thus, the phase error correction in conventional receivers often operate insufficiently, leading to reduced reliability of the overall system, and reducing the settling time of the receiver.